Comparative histological and biochemical studies were conducted with the flight muscles and mitochondria of several insect species to determine any correlation which exists between muscle structure and the metabolic activity of their mitochondria. Light microscopic examinations were used in the subcellular observations of these muscles. These included direct observations of freshly isolated material (wet mounts), as well as permanent preparations of transverse sections of some muscle fibers. Histological results were presented in a table which includes the muscle type, as well as the fiber, fibril, and mitochondria sizes of several insect species. The presence and abundance of mitochondria and their arrangement in straight rows between the fibrils, was shown to be common in the flight muscles of all insects investigated (except in the female cockroach, Periplaneta americana). In support of these results, a series of microphotographs showing a comparison of the features studied is presented. / Ph. D.